Low-impedance radio circuit



LOW IMPEDANCE RADIO CIRCUIT Filed June 7 1925 IUDIOIMPLIFIEI? gwventocfieorga K. Gann Patented Apr. 7, 1931 UNITED STATES PATENT OFFICE GEORGE K. GANN, OF ST. PAUL, MINNESOTA, ASSIGNOR, BY ASSIGNMENTS, TO PROGRAM SERVICE COMPANY, OF ST. PAUL, MINNESOTA, A CORPORATION OF DELA WARE LOW-IMPEDANCE RADIO CIRCUIT- Application filed June 7, 1926. Serial No. 114,207.

The present invention relates to the transmission of programs, such as music, news, general information, etc., over wires.

In the transmission of programs by wires,

especially when these wires are a part of a telephone system, or where the wires carrying the'program current are closely adjacent to other wires used for telephone or similar purposes, it is desirable to keep the voltage of the currents carrying said program as low as possible. This is desirable since where comparatively high voltages are used in the carrying of such programs by wire cross talk is apt to be produced between said program carrying conductor and a conductor used for telephonic purposes which is electrically sepa- -ated from but closely adjacent to said program carrying conductor for a portion of its length. This causes the program to be audible in the other telephonic circuit, thereby interrupting the normal use of said other circuit.

This objectionable cross talk exists when speech carrying current in one pair of conductors produces a current in a second pair of conductors carrying speech carrying currents of suflicient strength to produce audible sounds in the telephone receiver of the second circuit. The greater the voltage in the first circuit, the greater ordinarily will be the amount of cross talk induced into the second circuit.

It has heretofore been necessary, in order to deliver sufficient current to a program receiving device over a line, to send the program over the wire at a comparatively high voltage so that ample sound volume would be produced by the program receiving device. The voltages of such a program transmitting system as thus transmitted are ordinarily much higher than those ordinarily used in a telephone system. This high voltage has been necessary since the program receiving devices, such as loud speakers, head receivers, and similar devices, are of the high impedance type such as are ordinarily used in the normal reception of radio programs where the program is not sent out over a line. Furthermore, it has been found that in using voltages even as high as those employed in ordinary telephone practice that certain tones of sound, by varying the resistance of the transmitter causes distortion of the volume of the sound when reproduced by the receiving device. It is, therefore, herein contemplated to employ transmitting currents having voltages lower than those normally used in telephone'practice to prevent undue loudness of certain tones.

An object of the present invention is to distribute a voice carrying current of coni-' paratively higlr voltage from a program transmitting station to a. number of program transmitting lines, so that each line will carry a proportional part of the current from said program transmitting station, the voltage on said line being thus reduced to a fractional part of the voltage of the transmitting station, each of said lines being connected to a low impedance program receiving device con trolled by a compensating control device.

In order to attain this object there is provided in accordance with one feature of the invention, a program transmitting station having conductors with sound carrying current therefrom connected through the primary coils of a pluralityof transformers. the windings of which are preferably of the ratio of one to one. The secondary coil of each transformer is connected to a line which. at its other end, is connected through suitable volume control means to a low in'ipedam'e receiving device, such as a loud speaker.

These and other features of the invention not specifically mentioned will be more fully brought out in the followingdescription and the accompanying drawing, wherein:

The figure illustrates a schematic view of a circuit arranged in accordance with the present invention.

Referring to the drawing in detail, a pair of conductors 1 are connected to a suitable program transmitting station, not shown, and may be connected to the input terminals of an audio amplifying device 2. To the output terminals of the audio amplifying device" a second pair of conductors 3 and 4 are connected. These conductors are connected as shown through the primary coils 5 of a plurality of transformers A, which are preferably of the ratio of one to one. The secondary coil 6 of each of the transformers is connected to a pair of line conductors 7 and 8, which are connected at their other ends, through a suitable volume control device C, to a low impedance typereceiving device, such as a loud speaker 9. The conductor 7 is one of a plurality of telephone conductors 10 arranged adjacent thereto. The current across the conductors 3 and 4 is introduced through the transformers A to the conduc tors 7 and 8.

In the present instance, there being five transformers, the voltage on the line con ductors7 and 8 would be equal to one-fifth of the total voltage across the terminals 3 and 4, less the transformer and conductor losses which would naturally be incident to the process. If the voltage across the conductors, as at B, were,1for example 20 volts, the voltage across to each of the lines 7 and, 8 would be something less than 4 volts, or one-fifth of said current at 13. Since the ordinary voltage used in a telephone system ranges between 15 volts and zero, it is apparent that voltages well within the allowable range for telephone transmission may be used for the transmission of programs in accordance with the present invention, thereby completely eliminating any danger of.

cross talk which has heretofore been a most serious obstacle to such program transmission. In addition to the elimination of cross talk, the use of low voltage and low impedance receiving devices make for uniform sound reproduction without distortion of 1 sound tones which unduly reduce the resistance in the transmitter.

I claim:

1. In a system for transmitting programs over conductors adjacent conductors of communication circuits, an amplifier having an out ut voltage greater than the voltage of said communication circuit, means for preventing inductive interference or cross-talk between the program-carrying conductors and the communication conductors, said means comprisinga plurality of transform-- ductors comprising a pluralit of transformers having their primary win ings connected in series with the output circuit of said audio amplifier, lines connected to the secondary windings of said transformers, receiving devices having an impedance lower than the impedance of the telephone receiv ers in said telephone circuits operatively connected to said lines, and control means in said lines for maintaining a uniform load across the lines irrespective of the current passing through said receiving devices.

3. In a system for transmitting programs over conductors which are disposed adjacent conductors of communication circuits, an amplifier having an output voltage greater than the voltage of said communication circuit, means for preventing inductive interference or cross-talk betweenthe program carrying conductors and the communication conductors, said means comprising a plurality of transformers each having a winding thereof connected in series with the output circuit of said amplifier and a line connected to each of said tarnsformers to receive a portion of the current of said amplifier.

4. In a system for transmitting programs over conductors which are disposed adjacent conductors of communication circuits, an amplifier having an output voltage greater than the voltage of the communication circuit, a series of distantly located receiving devices, said program conductors extending between said receiving devices and said am."

lifier and a transformer systenrinter osed etween the output of said amplifier an said several program conductor-s adapted to impress upon said conductors currents at sufciently low potential to prevent inductive transfer of energ from said program-conductors to said a jacent conductors of communication circuits. In testimony whereof I afiix GEORGE signature.

I? GANN.

ers having a winding thereof connected in series with the output circuit of said amplifier, a line connected to each of said transformers to receive a portion of the current of said amplifier, a receiving device having an impedance lower than the impedance of the communication receiver and operatively connected to said line and control means connected to said line andto said receiving device.

2. In a system for transmitting programs over conductors adjacent conductors o telephone circuits, an audio amplifier having an output voltage greater than the voltage of the telephone circuit, meansfor preventing cross talk between the program carrying "conductors and the telephone carrying con- 

